1. Field of the Invention
The present invention relates to a linear direct current motor commonly used for moving an object to be moved with high accuracy in, for example, a motion mechanism such as a machine tool or industrial robot, and more particularly, to a brushless type of linear direct current motor.
2.Description of the Prior Art
FIG. 1 shows a drive unit containing a linear direct current motor of the prior art. Furthermore, this drive unit has a guide unit for guiding an object added to a linear direct current motor.
As shown in the drawing, this drive unit has a long base member 1 and moving body 2 which moves along said base member 1. More specifically, a plurality of rollers (not shown) are provided on moving body 2, and these rollers roll over a track (not shown) formed along the lengthwise direction in base member 1.
On the other hand, the linear direct current motor which composes the above-mentioned guide unit together with a drive unit is composed in the manner described below.
Said linear direct current motor is composed of a primary side, equipped with a large number of armature coils 5 arranged in a row in the lengthwise direction of base member 1 on said base member 1, and a secondary side, having a field magnet 6 (see FIG. 2) attached to the bottom surface of moving body 2 so as to oppose each of said armature coils 5. As shown in the drawing, said field magnet 6 is magnetized so that a plurality, in this case 4, of N and S magnetic poles are alternately arranged in a row along direction P in which moving body 2 is to move, namely the lengthwise direction of base member 1. Furthermore, as shown in FIG. 2, if the width of one magnetic pole of field magnet 6 is taken to be Pm in this example, the open angle width of each armature coil 5 is set to the same Pm.
In the linear direct current motor of the above-mentioned constitution, by supplying a prescribed excitation current to armature coils 5, thrust is produced based on Fleming's right hand rule between the primary and secondary sides. For example, if base member 1, to which the primary side is coupled, is taken to be the stationary side, moving body 2, integrated into a single unit with the secondary side, is moved by this thrust.
However, in the linear direct current motor as described above, it is important to systematically supply an excitation current to each armature coil to maintain as constant a thrust as possible regardless of changes in the position of the primary side with respect to the secondary side. Continuing, the following provides an explanation of the constitution pertaining to this supply of power.
As shown in FIG. 3, magnetic pole discrimination elements in the form of Hall effect elements 8a through 8f are respectively arranged in the vicinity of each armature coil 5a through 5f (six armature coils are shown in FIG. 3 in this case, and these six armature coils are mutually distinguished by adding small letters of the alphabet from a through f to reference numeral 5 indicating armature coils in the explanation thus far for the sake of convenience in the explanation). In this example, each of Hall effect elements 8a through 8f is arranged corresponding to conductors 5a.sub.1 through 5f.sub.1 on one side among the conductors that contribute to thrust possessed on two sides by each armature coil 5a through 5f. These Hall effect elements 8a through 8f emit a signal (in the form a potential difference) corresponding to the lines of magnetic force emitted by each magnetic pole possessed by field magnet 6 when said field magnet 6 approaches. Electrical power is then supplied to the armature coil corresponding to the Hall effect element that emitted said signal based on that signal. On the other hand, this supply of electrical power is interrupted to the armature coil corresponding to a Hall effect element for which said signal has yet to be obtained or is no longer being obtained, thus enabling control to be performed.
Control of the supply of electrical power is performed in the manner described below based on said constitution.
In FIG. 3, the letters A through G indicate that field magnet 6 is located at each of the positions shown in the drawing corresponding to those letters.
To begin with, in the case field magnet 6 is located at position A, each of the magnetic poles of said field magnet 6 act on the four Hall effect elements 8a through 8d. However, the two Hall effect elements 8a and 8d, on which the ends of the magnetic poles of said field magnet 6 act, do not respond. Only the remaining two Hall effect elements 8b and 8c respond. Accordingly, the two armature coils 5b and 5c, which respectively correspond to these Hall effect elements 8b and 8c, are supplied with electrical power. In this state, since each conductor 5b.sub.1, 5b.sub.2, 5c.sub.1 and 5c.sub.2, which contribute to thrust and of which two each are possessed by each armature coil 5b and 5c, is not acted on by the boundaries of each magnetic pole or ends of the magnetic poles of field magnet 6, and do not end up becoming displaced from each magnetic pole, all of these four conductors generate thrust.
Electrical power is continued to be supplied to the prescribed armature coils in the same manner as described above when field magnet 6 is located at each of the positions of B through G shown in FIG. 3.
Although the above-mentioned linear direct current motor of the prior art operates by supplying excitation current to each armature coil in the manner described above, the following problems occur as described below.
Namely, when field magnet 6 is located at position A shown in FIG. 3, thrust is actually generated by four of the conductors that contribute to thrust possessed by each armature coil as previously described. However, the number of conductors that generate thrust when field magnet 6 is moved to the other positions of B through G changes, namely being 4, 3, 3, 4, 4 and 3 conductors, respectively. Thus, a constant level of thrust cannot be obtained at all times.